The present invention relates to a method and apparatus for encoding a binary data stream into a binary code stream suitable for magnetic recording.
With recent advanced technology of A/D and D/A conversion, and analog signal such as an audio signal is digitally recorded in a magnetic tape or a magnetic disc. For recording a binary data stream consisting of a train of binary data "0" and "1" formed by the A/D conversion of an audio signal, as in the case of the analog recording, it is digital-modulated or encoded into a binary code stream suitable for mangetic recording. In the magnetic recording, the magnetizing polarity is inverted according to the binary code "0" and "1". MFM, 3PM and 4-12 conversion are enumerated for the conventional encoding methods. In this digital recording, a high density recording is desired, as a matter of course. To realize the high density recording, however, the following conditions are required. The first condition is that a minimum distance between transitions of magnetization Tmin is satisfactorily large. The second condition is that a maximum distance between transitions Tmax is satisfactorily small. The third condition is that a window margin Tw is large. When the minimum distance Tmin is small, a waveform interference is nonnegligible in the playback signal, resulting in increase of detection errors in the recording code. For effecting a high density recording, it is necessary to extract a clock signal for the demodulation (decoding) from the signal. If the minimum distance Tmin is fixed, as the maximum distance Tmax is larger, the number of the magnetizing polarity inversions input to a clock signal extracting system is smaller. The result is increase of a jitter of the clock signal. Therefore, if the maximum distance Tmax is large, the detection errors of the recording code increases. A window margin Tw means a time taken for sensing one bit of the recording code. In the high density recording, because of the presence of the waveform interference, noise etc., fluctuations in the phase of the reproduced waveform tends to be large. Therefore, when the window margin Tw is small, the detection error also tends to occur.
The above three conventional encoding methods are tabulated with respect to the above three conditions, as shown in Table 1.
TABLE 1 ______________________________________ Coding method Tmin Tmax Tw ______________________________________ MFM Tb 2 Tb 0.5 Tb 3PM 1.5 Tb 6 Tb 0.5 Tb 4-12 conv. 2 Tb 8.33 Tb 0.33 Tb ______________________________________
In the above table, Tb is a duration of one bit of the original data.
As seen from Table 1, none of the methods perfectly fullfills the above three conditions. When comparing with the MFM earliest developed, the 3PM is improved 1.5 times in the minimum distance Tmin, but is inferior 3 times in the maximum distance Tmax. In the 4-12 conversion, the minimum distance Tmin is doubled, but the maximum distance Tmax is inferior four times or more. As for the window margin Tw, it is preferable that it is large. However, an insufficiency of the window margin Tw could be compensated to some degree by reducing a variation of a speed of the recording medium in a playback mode to a minimum and by improving the accuracy of the clock extracting system. Consequently, it is safe to say that the 4-12 conversion method is most suitable for the high density recording, except the condition of the maximum distance Tmax.